This invention relates to a biomedical electrode. In particular, it relates to a biomedical electrode for use in the prolonged monitoring of a bio-electrical signal in a wet environment. In the preferred embodiments it relates to a biomedical electrode for use in the continuous monitoring of a foetal heart rate during labour.
Biomedical electrodes for external monitoring of electrical signals in humans are known in the art. Adhesives suitable for securing such electrodes to human skin are also known in the art. These adhesives are electrically conductive creams, pastes or gels applied directly to dry skin, thus forming an interface with the electrode. The Patent literature discloses several electrode designs incorporating the conducting adhesive as a coating on the electrode surface (for example, U.S. Pat. Nos. 4,674,512, 4,391,278, 4,125,110 and 4, 391,278). Removal of a release paper or liner allows direct attachment of the electrode to dry skin by application of light pressure. Thus, this type of design requires a pressure-sensitive adhesive. Pressure-sensitive adhesives may be produced conventionally by compounding an elastomer with a tackifying resin, or alternatively by using polymers which are inherently pressure-sensitive such as the polyacrylates and polyvinylether adhesives. For use in securing biomedical electrodes, a compound such as sodium chloride is also required in order to confer electrical conductivity on the adhesive. A further requirement is that the cohesive force of the adhesive, a measure of its structural integrity, should be greater than its adhesive force so that the electrode can be removed from the substrate without leaving an unacceptable residue.
Although the chemical nature of conducting, pressure-sensitive adhesive coatings for biomedical electrodes is wide, adhesion is lost in all such cases when the substrate and/or the environment is wet. Loss of adhesion in these cases is due to the adhesive absorbing moisture and swelling. Consequently, the electrode becomes detached from the skin surface. In addition, water effectively acts as a lubricant between skin and pressure-sensitive adhesive, preventing full bond strength from being developed and leading to immediate or rapid bond failure. The properties of conventional, pressure-sensitive adhesives known in the art may be found in Satas, D (Editor): Handbook of Pressure-Sen- Technology, Van Nostrand Reinhold, New York, 1982.
Biomedical electrode systems with pressure-sensitive adhesive interfaces are used, for example, in the monitoring of electrical activity of underlying muscles or a heart. In the case of muscles, the resulting signal being recorded is known as an electromyogram; in the case of the heart, the signal is known as an electrocardiogram.
A disadvantage of pressure-sensitive electrode adhesives known in the art is their inability to cope with exposure to significant amounts of moisture. Under very wet conditions adhesion is lost as the adhesive absorbs moisture and swells, with a consequent failure of signal monitoring.
Moisture activated adhesives are known in the art. Such adhesives are presented in the dry state and only exhibit adhesive properties when moistened, for example, paper labels or postage stamps coated with a dried gum. A disadvantage of such wet-stick adhesives is that they must be pre-moistened before being applied to the site of use.
A particular example of a biomedical electrode system which is required to function in a wet environment is an electrode used in the monitoring of human foetal heart rate during labour. The human foetus in the birth canal is surrounded by a considerable volume of aqueous fluid, about one liter. The foetal skin is also lightly coated with a protective material, the vernix. Under these conditions, an electrical sensor cannot be secured to the foetal skin by means of a conventional, pressure-sensitive adhesive. In May and Mahlmeister (1990): Community Maternity Nursing. Nursing Process and the Childbearing Family, 2nd. edn:., Lippincott, Pa., there is disclosed a design for a foetal scalp electrode in which the interface between the foetal skin and the signal conducting substance is made by means of a metal clip secured to the foetal head. The use of such electrodes, which detect the electrical energy produced during each cardiac cycle, is well-known in the art. The design suffers from several disadvantages:
(a) It causes trauma to the foetal skin, which is punctured by the metal clip. The extent of foetal distress caused by this trauma has been widely reported. PA1 (b) Care must be taken during application to ensure that the electrode is placed over the parietal bone of the foetal scalp and not over the anterior or posterior fontanelles. PA1 (c) There is a risk of trauma to such areas of the foetus as the spine, eyes etc. if the electrode is incorrectly applied. PA1 (d) It has been reported in the literature that the sharp metal clip frequently punctures the skin of the person inserting the electrode. This has serious implications if the mother is HIV positive or is a carrier of other transmittable diseases such as hepatitis. PA1 (e) It is comparatively expensive. PA1 (a) It will give a clear, low noise signal. PA1 (b) It will be disposable, requiring no special preparations to be made before application. PA1 (c) It will have no sharp components. PA1 (d) It will not puncture the foetal skin. PA1 (e) It will not present any risk to attending medical and other staff. PA1 (f) It will attach directly to the foetal skin by an adhesion process. PA1 (g) It will remain reliably attached throughout the period of labour, even in the presence of aqueous fluid. PA1 (h) It will be easy to remove. PA1 (i) It will leave no toxic residue on, or cause any damage to, the foetal skin. PA1 (j) It can be fabricated by conventional mass production techniques known in the art. Consequently, it will be comparatively inexpensive. PA1 (k) It can be attached to a foetal monitor and recording device by conventional means known in the art.
Thus, there exists a need for a foetal monitor electrode system which will possess one or more of the following characteristics:
The invention, therefore, provides a biomedical electrode device comprising an electrically insulating substrate, an electrode on the substrate, and a moisture-activated electrically conductive bioadhesive layer on the electrode, the bioadhesive layer having an adhesion of between 50 and 500 g/cm.sup.2 and a water content of less than 25% w/w.
In this context, adhesion may be defined as the state in which two bodies, in the form of condensed phases, are held together for extended periods by interfacial forces. These forces may involve covalent bond formation, mechanical, chemical or physical interactions. When one or both adherents are of a biological nature the process is known as bioadhesion. Of particular significance is Type III bioadhesion in which artificial material is made to adhere to a biological substrate. Bioadhesion is a relatively new area of study. Since there is no overall accepted theory of bioadhesion, the development of bioadhesives has tended to be empirical. In many cases adhesion is to epithelium coated with a thin gel layer of mucus. Mucins, viscoelastic glycoproteins, constitute the main components of mucus. Adhesion to this mucosal gel layer is referred to as mucoadhesion. Therefore, viscoelastic polymer formulations for use as bioadhesives have to be tailored to the nature of the biological substrate.
The surface of a biological substrate carries a negative charge. Therefore, polycations present the best opportunity for successfully formulating bioadhesives. However, these tend to disrupt cell membranes when used in isolation, raising the important issue of adhesive biocompatibility. For this reason bioadhesive hydrogels are preferred. Hydrogels are cross-linked hydrophilic molecules, either synthetic or naturally occurring, which have the ability to swell in water without dissolving, and to retain water within their structure. These properties confer a high degree of biocompatibility on hydrogel polymers, hence their extensive applications in medicine. Therefore, in one particular embodiment of the invention, the bioadhesive, moisture-activated electrically-conducting interface is formed from one or more hydrogels, together with the addition of a conducting material known in the art, typically an ionic salt, and with the further addition, where necessary, of a plasticising agent capable of rendering the dried interface pliant and conformable. If the bioadhesive formulation possesses sufficient inherent electrical conductivity, the salt component may be omitted.